1. Field of the Invention
The present invention relates to an organic EL display device, and more particularly to an organic EL display device which prevents deterioration of an organic EL layer due to moisture and exhibits excellent life characteristics.
2. Background Art
In an organic EL display device, an organic EL layer is sandwiched between a pixel electrode (lower electrode) and an upper electrode, a fixed voltage is applied to the upper electrode, and emission of light from the organic EL layer is controlled by applying a data signal voltage to the lower electrode thus forming an image. The data signal voltage is supplied to the lower electrode via a thin film transistor (TFT).
An organic EL display device is classified into a bottom-emission-type organic EL display device in which light emitted from organic EL layers is taken out in the direction of a glass substrate on which the organic EL layers and the like are formed and a top-emission-type organic EL display device in which light emitted from organic EL layers is taken out in the direction opposite to a glass substrate on which the organic EL layers and the like are formed. The top-emission-type organic EL display device has an advantage that the respective organic EL layers can ensure a large light emission area thus increasing the brightness of a display.
When moisture is present in an organic EL material used in an organic EL display device, the light emission characteristic is deteriorated and hence, when the organic EL display device is operated for a long time, portions of the organic EL material which are deteriorated with moisture do not emit light. These portions appear as dark spots on a display region. The dark spots grow with time and become a defect of an image.
To prevent the generation or the growth of the dark spots, it is necessary to prevent the intrusion of moisture into the organic EL display device or to remove the intruded moisture from the organic EL display device. Accordingly, an element substrate on which an organic EL layer is formed is sealed by a sealing substrate thus preventing the intrusion of moisture into the inside of the organic EL display device from the outside. On the other hand, to remove moisture intruded into the inside of the organic EL display device, a desiccant is arranged in the inside of the organic EL display device. This organic EL display device is referred to as a hollow-sealed-type organic EL display device.
The hollow-sealed-type organic EL display device has drawbacks such as difficulty in adjusting a gap between the element substrate and the sealing substrate and difficulty in adjusting pressure in a sealed space inside the organic EL display device. For example, the sealing material is made of an ultraviolet-ray curing epoxy resin. This resin cannot completely interrupt the intrusion of moisture from the outside and hence, the intruded moisture is diffused in a hollow portion. Accordingly, it is difficult to effectively protect an organic EL layer from moisture in case of the hollow-sealed-type organic EL display device.
To cope with such drawbacks attributed to the hollow sealed structure, there has been known a solid sealing technique which is disclosed in JP-A-2004-157517 (patent document 1). The solid sealing technique is a technique in which a space defined between an element substrate and a sealing substrate is filled with a liquid or a solid body such as an adhesive material. Patent document 1 discloses the constitution in which the element substrate and the sealing substrate which has a recessed portion are laminated to each other and, thereafter, a space defined between the element substrate and the sealing substrate is filled with silicone oil. Since silicone oil works as a stress buffering material, the substrate is hardly broken even when the substrate is curved or warped by an external force.
However, even when silicone oil is dehydrated for a long time, it is difficult to dehydrate silicone oil to an extent that silicone oil is practically applicable to an organic EL display device and hence, with a lapse of time, moisture in silicone oil precipitates and intrudes into an organic EL layer thus lowering a lifetime of the element.
JP-A-2005-533919 discloses another related art of the solid sealing structure. To be more specific, a desiccant is supplied in a state where the desiccant is dissolved in an adhesive organic solvent, and a chemical reaction is generated by heat or light thus making an element substrate and a sealing substrate adhered to each other.
The technique disclosed in patent document 1 has following drawbacks. That is, the technique uses a liquid as a desiccant and hence, the technique requires the structure for injecting the liquid in the inside of the organic EL display device. Further, the technique requires a liquid injecting process and hence, manufacturing steps become complicated.
On the other hand, the technique disclosed in patent document 2 has following drawbacks. That is, when the desiccant is supplied in a sol state where the desiccant is impregnated in a low-molecular organic solution used as an adhesive material, an organic solvent is vigorously evaporated. The vigorous evaporation of the organic solvent implies that an adhesive strength is strong. The rapid evaporation, on the other hand, implies that an organic gas is liable to remain in the organic solvent correspondingly. This organic gas, as a result, shortens a lifetime of the organic EL display device. Accordingly, in the related art, it is necessary to form a silicon nitride film having a large thickness as a background film of a desiccant containing adhesive material. Further, there also exists a drawback that the solvent having low viscosity such as a solvent in a sol state is difficult to handle.